In our computers, there is one EPROM chip called BIOS, which is situated on the motherboard or main board of our computers. When we power on, the processor starts executing a program from BIOS. The program from BIOS, does a power on self-test such as checking memory and other peripherals. Then the BIOS program initializes the basic hardware required for PC operation, such as initializing the PCI bus, video devices, and similar.

Finally, BIOS checks the boot device sequence and queries the first boot device. This BIOS program then reads the master boot record of the first boot device, which is normally a hard disk, USB device, or DVD. Once BIOS reads the master boot record of the first boot device, then the boot loader is started. The boot loader reads kernel binary and copies it in the RAM memory. The boot loader checks if the kernel binary is clean and not corrupt. If the integrity check is good then it uncompresses the kernel in the RAM. The bootloader then calls the start_kernel()function, which is a part of kernel. Once the start_kernel()function is called, the kernel is started.

The kernel then initializes the subsystems of kernel such as process management, filesystem, device drivers, memory management, network management, and similar other modules of the kernel. Then, it mounts the root file system, and kernel creates the first process called init. This init process reads the /etc/inittab file. In inittab, the run level information is stored. As per this information, the operating system is initialized process init.

The typical /etc/inittab content will be as follows:

$ cat /etc/inittab

Output:

# Default runlevel. The runlevels used are:
#   0 - halt (Do NOT set initdefault to this)
#   1 - Single user mode
#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)
#   3 - Full multiuser mode
#   4 - unused
#   5 - X11
#   6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:

In the preceding line, the number 5 after ID specifies that the system should be started in run level 5. It means that the system should be started in X11, such as a graphical user interface. We will study more about run levels in the next section.

Nowadays, many distributions have modified the boot-up sequence. They have removed the /etc/inittab file and used different applications to customize the boot-up process.

There are seven run levels. The system will be started in run level 1 to 5. Run level 0 is used for shutting down the system. Run level 6 is used for rebooting the system. The graphical user interface is started in run level 5. The following is the summary of different run levels:

We need to be in the root-user mode to use the init command.

If we give the following command, then the system will shutdown:

# init 0

To reboot the system use the following command:

# init 6

If the system is running in the command-line mode, and if you want to start your server in the graphical user mode, then use the following command:

# init 5

In the Linux system, the following folders will be present in the /etc/ folder:

Every run level folder will have script names starting either with S or K. When starting the system, the scripts with names starting with S are called one after another. When shutting down, all the script names starting with K are called one after another.

For example, if the system has to be started in run level 5, then initially all the scripts from the rcS.d folder will be called, then all the scripts from rc5.d will be called. Finally, the rc.local script will be called.

The content of /etc/rc.local is as follows:

$ cat /etc/rc.local

Output:

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change
# the execution bits.
#
# By default this script does nothing.
exit 0

We can add our customization commands before the exit 0 line in the preceding rc.local script.

Before any user is logged in, the mentioned scripts will be called. After this, user login initialization will be started. This is explained in the following sessions.